1. Field of the Invention
This invention relates generally to breathing ventilators, and more particularly relates to a programmable safety circuit for a lung ventilator that provides ventilation pressure relief when ventilation pressure exceeds a set maximum pressure.
2. Description of Related Art
Medical ventilators are generally designed to ventilate a patient's lungs with breathing gas to assist a patient in breathing when the patient is somehow unable to adequately breath without assistance. Pressure assistance can be instituted, for example, when the patient has already begun an inspiratory effort. With such a system, it is desirable to immediately increase the pressure after a breath is initiated in order to reach a target pressure. This rise in pressure causes flow to be initiated in the patient airway which supplies breathing gas to the patient's lungs. Conventional pressure controlled ventilator systems typically implement a gas flow control strategy of terminating or reducing breathing gas flow when the target pressure is reached, to limit patient airway pressure.
However, such a control strategy can result in over-pressurization of the patient's lungs, particularly when high pressure gasses are used for blending the breathing gas mixture. When the over-pressurization is sustained, the patient's lungs can be subjected to excessive pressure for an entire inspiration portion of a breath cycle. When this occurs, the possibility exists that the patient will be harmed by a higher than desirable pressure in the lungs. Such overpressure can, for example, rupture sutures or blood vessels of a patient that has recently undergone thoracic or abdominal surgery. Similarly, frail or infirm patients, such asthmatic or emphysemic patients, can also be harmed if airway pressure is excessive.
Conventional ventilators have also typically limited ventilator pressure to a maximum by a pressure relief valve or safety valve with a fixed maximum rated pressure. However, such systems do not provide for a setting of the desired maximum pressure, which may be considerably lowered for infants or intensive care patients. Patient airway pressure can be also be actively controlled by an exhalation valve, although in some ventilators such exhalation valves can be simply closed during inspiration in order to achieve a set maximum ventilator pressure, so that it is possible for excessive pressure buildup to occur during inspiration pressure assistance. Ventilator system malfunctions can also result in overpressurization in the event of failure of a high pressure gas delivery valve controlling introduction of one of the breathing gas components into a high pressure blender.
For example, one known piston based lung ventilator utilizes a rolling-seal piston of low inertia and low frictional resistance for delivery of breathing gas, which is mixed in the piston chamber. For mixing of the breathing gas in the piston chamber, the piston chamber has an inlet connected to the airway of the patient, and a one-way valve allows air to enter the piston chamber during the exhalation phase of the respiratory cycle. Another gas inlet to the piston chamber, controlled by a solenoid valve, allows introduction of a desired gas mixture into the piston chamber. A valve for introducing the gas mixture is opened during expiration as the piston reciprocates to a baseline position. The oxygen content of the inspired gas can also be enriched by admitting a continuous flow of oxygen into the piston chamber through another inlet. However, the gases mixed in the breathing gas are supplied at high pressure. If a valve controlling the introduction of high pressure oxygen or air fails, breathing gas can be provided to a patient at an excessive pressure.
It would be therefore be desirable to provide a programmable lung ventilator safety circuit useful with virtually any type of ventilator delivering breathing gas under pressure, to limit the ventilator airway pressure to a maximum pressure appropriate for each particular patient, so that even if the ventilator should fail for some reason to provide ventilator pressure below an acceptable maximum, breathing gas will not be provided to a patient at an excessive pressure. The present invention meets these needs.